This invention relates to data communication satellites, and more particularly to data communication satellites capable of providing two-way multi-media communications with residential and small business satellite terminals.
Existing trends in satellite communications suggest that a shift is occurring to offer services to multi-media traffic. Current satellite data services are geared to users who generate steady-stream data. Although these services can be used to transport multi-media traffic, compromises on the channel quality and efficiency must be made. If the data channels are fitted to operate at peak data rates, the channel usage becomes very inefficient. If the channel is sized to the average traffic rate, clipping will result. A means of supporting bandwidth-on-demand is therefore being considered to optimize multi-media services.
Since these new traffic sources are bursty by nature, the ratio between peak and average channel use can be in the order of two magnitudes. The efficient support of multi-media traffic sources will demand that spare capacity be shared between all users and applications. Because of the non-uniform nature of the expected traffic, fast-packet switching techniques have been identified as the best means of switching traffic amongst the user population. For example, data communication satellites are targeted at supporting multi-media traffic efficiently, which means handling traffic with varying Jitter Tolerance, Cell Loss Tolerance, Circuit Set-up (Access), Latency and Source Burstiness.
Some classes of traffic cannot tolerate cell loss or jitter, but have low burstiness and can tolerate relatively long call set-up delays. (Examples are standard telephones, stream data and Constant Bit Rate (CBR) video). These require emulating a conventional fixed circuit assignment, and are classified as Real Time (RT) data.)
At the other extreme is packet data, which has very high burstiness (typically 400:1 ratios of burst to average rate) and requires a short access latency, but can tolerate transmission jitter and cell loss (using ARQ Recovery Protocols). Carrying this class of traffic efficiently requires bandwidth-on-demand protocols with a short response time to individual bursts (less than one second). Such traffic is classified as Jitter Tolerant (JT) data. There is a third class, representing compressed/statistically multiplexed voice and Variable Bit Rate (VBR) video which requires a guaranteed no-loss connection which has moderate burstiness, can tolerate a relatively long circuit set-up time but which requires very short response (less than a few milliseconds) to transient burst. Because this class covers compressed data, with a significant decompression processing delay, it should be able to tolerate minor jitter in transmission, and is referred to here as Near Real Time (NRT) data.
To handle these diverse requirements over a geo-stationary satellite, with its inherent long round trip propagation delay of about a quarter-second, requires a very efficient uplink access protocol and responsive capacity assignment algorithm. These and other fundamental network requirements need to be addressed in order to efficiently support multi-media traffic. With the increasing amount of information required to support each multi-media user, efficient use of network capacity, even in terrestrial networks, is required as evidenced by the emergence of the ATM standard for multiplexing and switching. For a satellite network the efficient use of bandwidth is even more imperative due to physical limitations and cost of establishing that resource in space orbit.
Thus, satellite-based multi-media data services can only be implemented efficiently if the concept of bandwidth-on-demand is optimized.
The concept of xe2x80x9cbandwidth-on-demandxe2x80x9d simply refers to allocating bandwidth to a user when it is required, and in its most dynamic form, matching the instantaneous bandwidth variation of the source traffic profile. This in turn permits a greater efficiency of bandwidth utilization by statistical multiplexing of traffic, thereby increasing the number of users that the system can simultaneously support. Although Demand Assignment, Multiple Access (DAMA) systems have been offered in the past to provide bandwidth-on-demand on a call-by-call basis, i.e. circuit assignment, these have been found to be unsuitable for offering bandwidth-on-demand for a satellite-based fast-packet switch.
Accordingly, a need exists for providing a satellite data communication system and method of operating the same in order to provide switch transport services for multi-media networks, wherein access schemes, protocols and switching techniques are combined to offer optimized dynamic bandwidth-on-demand on a packet-by-packet basis.
It is therefore an object of the present invention to provide a data communication satellite system capable of carrying multi-media traffic with varying jitter and cell loss tolerances, circuit set-up time, latency and source burstiness.
Another object of the present invention is to provide a data communication satellite system with an efficient uplink access protocol with responsive capacity assignment algorithm.
Another object of the present invention is to provide a data communication satellite system which make use of an on-board ATM-like switch capable of providing bandwidth-on-demand.
Yet another object of the present invention is to provide a data communication satellite system which makes use of a beam grouping management technique to enable flexible resource allocation.
Another object of the present invention is to provide a data communication satellite system which utilizes rain fade countermeasures for uplink and downlink transmissions.
According to one aspect of the present invention, there is provided in a satellite communication system carrying data traffic between a data communication satellite and plurality of ground terminals, a method of allocating bandwidth resources amongst a number of spot beams used by said data communication satellite to establish uplink and downlink communication carriers with said ground terminals, comprising the steps:
assigning a predetermined frequency reuse pattern to a number of spot beams forming a geometric configuration of beam groups covering said plurality of ground terminals;
distributing the available bandwidth between each beam group such that a downlink carrier of predetermined bandwidth is shared between spot beams forming said beam group; and
allocating a predetermined amount of transmission time on said downlink carrier to each spot beam, according the traffic density and bandwidth requirements of each spot beam
In another aspect of the present invention there is provided an improved satellite communication system for carrying data traffic between a data communication satellite and a plurality of ground terminals, each ground terminal being operable in a spot beam for communicating with said data communication satellite over uplink and downlink carriers, wherein the improvement comprises:
each ground terminal being operable to communicate with said data communication satellite via an uplink MF-TDMA carrier having a signal bandwidth which is distributed pseudo-statically in an integral number of carriers with a minimum capacity of one, carrier in each spot beam; and
each ground terminal being operable to communicate with said data communication satellite via a downlink TDM cell burst which vary dynamically in length from frame to frame according to the traffic requirements of each spot beam.
In yet another aspect of the present invention there is provided in a satellite communication system for carrying data traffic between a source terminal and a destination terminal via a data communication satellite having an on-board switching system, a method of routing data packets through said on-board switching system, comprising:
receiving, at said on-board switching system, a data packet from said source terminal, said data packet having an information field and traffic routing fields;
decoding a DownLink address (D/L) from said traffic routing fields to determine the destination of said received data packet;
transmitting said received data packet to an appropriate downlink carrier according to said decoded D/L address;
receiving at said destination terminal, said data packet; and
decoding a Virtual Circuit Identifier (VCI) from said traffic routing fields to identify which cells of said data packet are applicable to said destination terminal.
In yet another aspect of the present invention there is provided in a satellite communication system for carrying data traffic between a source terminal and a destination terminal via a data communication satellite having an on-board switching system, a method of scheduling cell priority of data packets carried on a downlink carrier established between said data communication satellite and said destination terminal, comprising:
receiving, at said on-board switching system, a data packet from said source terminal, said data packet having a cell type field indicative of cell priority to be allocated to the received data packet;
decoding said cell type field from said data packet to determine the cell type; and
transmitting said received data packet via an appropriate downlink carrier to a destination terminal according to the cell type identified with said data packet.
In yet another aspect of the present invention there is provided in a satellite communication system carrying data traffic between a data communication satellite and plurality of ground terminals, a method of dynamically reducing the effects of rain fade on uplink and downlink communication carriers established between said data communication satellite and said ground terminals, comprising the step:
receiving a transmitted signal via one of said uplink and downlink communication carriers;
measuring the power level of the received signal;
identifying that said transmitted signal is being subjected to rain fading;
determining the level of rain fading; and
applying a rain fade countermeasure to said transmitted signal.